Publication Date:
2022-05-25
Description:
Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 71 (2007): 1170-1182, doi:10.1016/j.gca.2006.11.017.
Description:
Sulfide sulfur in mid-oceanic ridge hydrothermal vents is derived from leaching of basaltic-sulfide and seawater-derived sulfate that is reduced during high temperature water rock
interaction. Conventional sulfur isotope studies, however, are inconclusive about the mass-balance
between the two sources because 34S/32S ratios of vent fluid H2S and chimney sulfide
minerals may reflect not only the mixing ratio but also isotope exchange between sulfate and
sulfide. Here, we show that high-precision analysis of S-33 can provide a unique constraint
because isotope mixing and isotope exchange result in different Δ33S (≡ δ33S – 0.515 δ34S) values
of up to 0.04 ‰ even if δ34S values are identical. Detection of such small Δ33S differences is
technically feasible by using the SF6 dual-inlet mass-spectrometry protocol that has been
improved to achieve a precision as good as 0.006 ‰ (2σ).
Sulfide minerals (marcasite, pyrite, chalcopyrite, and sphalerite) and vent H2S collected
from four active seafloor hydrothermal vent sites, East Pacific Rise (EPR) 9-10° N, 13° N, and
21° S and Mid-Atlantic Ridge (MAR) 37° N yield Δ33S values ranging from –0.002 to 0.033 and
δ34S from –0.5 to 5.3 ‰. The combined δ34S and Δ33S systematics reveal that 73 to 89 % of vent
sulfides are derived from leaching from basaltic sulfide and only 11 to 27 % from seawater-derived
sulfate. Pyrite from EPR 13° N and marcasite from MAR 37° N are in isotope
disequilibrium not only in δ34S but also in Δ33S with respect to associated sphalerite and
chalcopyrite, suggesting non-equilibrium sulfur isotope exchange between seawater sulfate and
sulfide during pyrite precipitation. Seafloor hydrothermal vent sulfides are characterized by low
Δ33S values compared with biogenic sulfides, suggesting little or no contribution of sulfide from
microbial sulfate reduction into hydrothermal sulfides at sediment-free mid-oceanic ridge
systems. We conclude that 33S is an effective new tracer for interplay among seawater, oceanic
crust and microbes in subseafloor hydrothermal sulfur cycles.
Description:
S. Ono thanks the Agouron Institute for financial support and funding from the NASA Astrobiology Institute and Carnegie Institution
of Washington for supporting the analytical costs. Funding for O. Rouxel is from the Deep Ocean Exploration Institute at WHOI.
Keywords:
Sulfur isotope
;
Multiple-isotope
;
Mass-dependent
;
S-33
;
S-36
;
Sulfur cycle
;
Hydrothermal
;
Vent
;
Mass-independent
;
Isotope fractionation
Repository Name:
Woods Hole Open Access Server
Type:
Preprint
Format:
application/pdf
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